Many-to-many

On an EF Core resource, add a many-to-many navigation as a field, and it just works:

public class Trip
{
    public int Id { get; set; }
    public string Name { get; set; } = "";
    public List<Tag> Tags { get; set; } = [];
}

resource.AllowEdit(edit => edit.Fields(fields =>
{
    fields.Add(trip => trip.Name);
    fields.Add(trip => trip.Tags);
}));

With Tag registered as a resource, the field gets a multi-lookup. Saving adds and removes the rows in the join table. The navigation doesn't need a setter.

Other editorsCopy Anchor Link

For a short list, checkboxes or chips are friendlier than a lookup:

fields.Add(trip => trip.Tags).UseCheckboxGroupEditor();
fields.Add(trip => trip.Tags).UseToggleGroupEditor(tags => tags.Appearance = ToggleGroupAppearance.Chips);

Those two and UseMultiLookupSheetEditor are the only editors that work here. A select picks one value, so it's left out and reported as a configuration problem.

The multi-lookup takes the same Field, Sheet and EnableCreate settings as the single lookup. Its Layout is Chips, List or Summary:

fields.Add(trip => trip.Tags).UseMultiLookupSheetEditor(lookup =>
    lookup.Field(field => field.Layout = MultiLookupSheetEditorLayout.List));

How manyCopy Anchor Link

[MinLength] and [MaxLength] on the navigation limit how many can be picked:

[MinLength(1), MaxLength(3)]
public List<Tag> Tags { get; set; } = [];

When it doesn't workCopy Anchor Link

This works when EF Core maps the join table without an entity type of its own. If you have a join entity, like TripTag with extra columns, or the related entity has a composite key, the field is left out. Use a form model with a list of keys, and a handler that saves the join rows:

fields.Add(form => form.TagIds).UseMultiLookupSheetEditor(lookup => lookup.UseResource<Tag>());

Showing themCopy Anchor Link

Columns and view fields show the related records as badges with BadgeDisplay. See showing related values.